The present invention relates to an improvement in a noise insulation structure for a vehicle floor section.
Noises reflected from road surfaces and noises radiated from an exhaust system are transmitted through a steel floor panel into a vehicle compartment. It is desirable that such transmission noises are as small as possible for comfort of a vehicle occupant. To this end, various attempts have heretofore been made in the past to take a variety of noise insulation measures for a floor-placed carpet (see Japanese Patent Publication No. 6-2988).
FIGS. 3A to 3C hereof illustrates a prior art noise insulation structure for a vehicle floor section.
FIG. 3A shows a schematic view of a vehicle body 101 of a vehicle, and a noise insulation structure embodied on a floor section b of the vehicle body 101 is described in detail below with reference to FIG. 3B.
In FIG. 3B, the prior art noise insulation structure for the vehicle section is comprised of a urethane layer 103 placed over a vehicle floor panel 102 made of steel, and a carpet 105 is placed over the urethane layer 103 via a mass layer 104.
FIG. 3C is a resilient modeling representation of the prior art noise insulation structure and shows the noise insulation structure of a one-way freedom system assuming that, in FIG. 3B, the mass layer 104 has a mass of m1 and the urethane layer 103 beneath the mass layer is made of a spring with a spring constant of k1. The noise insulation characteristic of the noise insulation structure of such a one-way freedom system is described below with reference to a subsequent figure.
FIG. 4 is a noise insulation performance curve of the prior art noise insulation structure, with the horizontal axis and the vertical axis indicating a frequency and a transmission loss, respectively. It appears that the larger the transmission loss, the higher will be the noise insulating performance.
A symbol f1 in the horizontal axis designates a primary resonance point and, in a high frequency range beyond the primary resonance point f1, the transmission loss increases at a gradient of 12 dB per one octave.
In the meantime, although the prior art structure forms the noise insulation structure of the one-way freedom system and a noise insulation effect of 12 dB per one octave can be expected, it is required for the vehicle to have a further increased noise insulation performance to effectively alleviate load noises arising from the rough road surface during a travel of the vehicle. Namely, the load noises arising from the rough road surfaces during the travel of the vehicle are transmitted through the floor section of the vehicle to reach the vehicle compartment and, so, it is highly required for the vehicle to undertake the load noise measure to provide a further improvement over the noise insulation structure of the vehicle floor section to obtain an increased noise insulation performance.
According to the present invention, there is provided a noise insulation structure, for a vehicle floor section, which comprises a floor panel made of steel, a urethane layer placed over the floor panel, a lower mass layer placed over the urethane layer, a low resilient material layer placed over the lower mass layer, an upper mass layer placed over the low resilient material layer, and a carpet placed over the upper mass layer.
The lower mass layer and the urethane layer beneath the same, and the upper mass layer and the low resilient material layer beneath the same establish a noise insulation structure of a two-way freedom system. Such a configuration enables an increased noise insulation performance of even 24 dB per one octave to be obtained. However, the mere presence of the noise insulation structure of the two-way freedom system suffers from a remarkable drop in the noise insulation performance especially at a secondary resonance frequency. For such measures, a low resilient material layer with a resilient rate lower than that of urethane is adopted to lower the secondary resonance frequency while locating urethane with a high adhesion property on the floor panel to cause an air gap to be dispensed with to preclude noise from being leaked. This results in a capability of exhibiting a high noise insulation performance of 24 dB per one octave in a range starting from a further low frequency area. As a result, the load noises arising from the rough road surfaces can be effectively attenuated to enable the vehicle to be provided with a further comfortable vehicle compartment.
The low resilient material layer set forth above may be comprised of, for instance, according to an example of an embodiment, a layer composed of fibrous material, a low resilient urethane layer and a hard felt layer.
The above urethane layer may be preferably formed with a concave portion so as to open upward in which the above lower mass layer and the above low resilient material layer are fitted whereupon the carpet is adhered thereto. With such a configuration, adhesion between the upper layer portion and the lower layer portion may be dispensed with, resulting in a capability of avoiding reduction in noise insulation performance as well as increase in cost-up factors.